1. Field of the Invention
The present invention relates to polyester resins suitable for the production of plastic containers, such as beverage bottles. Specifically, the present invention is directed to the use of a preblended additive in polyester synthesis reactions to reduce or eliminate unwanted color and/or improve bottle clarity, and to compositions including such preblended additive. More specifically, a cobalt/phosphorus compound or complex is formed and added at the end of esterification or at the beginning of polycondensation during polyester production.
2. Background of the Invention
Polyester resins are extensively used in the packaging industry and especially in the production of plastic bottles for containing a wide variety of liquids, including carbonated and non-carbonated beverages. In the manufacture of bottles, such polymeric resins are extruded and formed into chips. The use of chips in injection molding to make a bottle preform which then is reheated and blown into a mold to produce the final shape of the bottle, is well-known and established in the industry.
One problem that is often encountered in the bottle manufacturing process is the occurrence of a yellow color in the final polyester product. The yellow color in polyester articles results from undesirable side reaction products in the esterification and polycondensation reactions, which impart such yellow color. The yellow color of the polyester resin has resulted in the widespread usage of cobalt compounds as blueing agents in the manufacturing process. Blueing agents counterbalance the yellow color to produce a neutral bottle color. Cobalt acetate is the most common compound and when added in the range of 30-70 ppm (parts per million, weight) is generally sufficient to cover the yellowness in the resin or bottle. The use of cobalt as a blueing agent is more fully described in U.S. Pat. No. 5,623,047.
Although cobalt compounds are very effective against the yellowness problem sometimes cobalt can impart a xe2x80x9cblue hazinessxe2x80x9d to the product, which also is undesirable in the bottle manufacturing process. Microscopic analysis revealed that the haziness corresponded with the presence of particles 500 nanometers in diameter. The particles were identified as cobalt compounds or complexes.
In addition to using cobalt compounds as bluing agents, cobalt can also be used as an esterification catalyst to produce polyester resin, as described in U.S. Pat. Nos. 5,623,047 and 5,782,963. While it is generally recognized that cobalt can reduce the reaction time (enhancing the reaction rate) as an esterification catalyst, use of cobalt can yield resins of low clarity as described in U.S. Pat. No. 5,116,938, due to the aforementioned blue haziness characteristic. Additionally, the presence of cobalt in the polyester process may hinder the catalytic activity of manganese and lithium, which are often used as catalysts in the esterification reaction.
More specifically, polyethylene terephthalate (PET) is typically produced for the manufacture of plastic containers by the so-called DMT (dimethyl terephthalate) process or the TA (terephthalic acid) process. In the conventional DMT process, DMT and ethylene glycol (EG) are used as raw materials. The DMT and EG are reacted in a reaction vessel in the presence of an esterification catalyst at high temperatures, typically around 250xc2x0 C. The esterification catalyst can be any of various elements including manganese, zinc, calcium and cobalt. The esterification reaction produces the monomer and oligomers. It usually is necessary to arrest the activity of the esterification catalyst in the DMT process to prevent degradation of the resultant polymer and minimize further yellowness. It is known to add phosphorous compounds at the end of the esterification to sequester the catalyst to arrest their catalytic activity.
In the conventional TA process, TA and EG are reacted in a vessel without the addition of catalysts and this direct esterification occurs at high temperatures of about 250xc2x0 C. to produce monomer and oligomers. The monomer and oligomers are then polymerized by polycondensation under high temperature (250 to 300xc2x0 C.) and vacuum to produce PET.
It is a primary objective of the present invention to provide polyester resins that are transparent (i.e., have no haziness), and are substantially neutral in color (i.e., have no significant yellowness or blueness) and are suitable for bottle production.
Other objects, features and advantages of the invention will be more fully apparent from the following disclosure and appended claims.
The present invention provides polyester resins that are transparent (i.e., have no haziness), and have no significant yellowness or blueness, by use of a preblended cobalt-phosphorus complex or compound in the polyester synthesis reaction.
More specifically, the present invention provides an additive composition and process for the production of PET by a dimethyl terephthalate (DMT) or terephthalic acid (TA) process, wherein the final PET resins have good clarity and color characteristics. The PET resins hereinafter more fully described are employed for the manufacture of plastic containers such as bottles. The invention utilizes an improved additive that stabilizes the cobalt and substantially eliminates the xe2x80x9cblue hazexe2x80x9d which frequently results from the addition of cobalt in the esterification or polycondensation reaction. Additionally, the present invention yields a resin having good color and little or no haziness.
In application to bottle production, the polyethylene terephthalate produced in accordance with the present invention, by use of the preblended cobalt formulation, yields a clear non-hazy bottle, which exhibits minimal or attenuated color (yellow or blue).
In one aspect, the invention relates to a method of making a PET resin that is further polymerizable by solid-state polymerization and moldable to form a PET article that is transparent and substantially free of hazy and yellow or blue coloration, comprising producing a PET resin by a DMT or TA process, including formation of the monomers, and subjecting the monomers to polycondensation reaction to form PET resin, wherein before the polycondensation reaction, an effective amount of a complex or compound comprising a cobalt compound and a phosphorus compound are added to the reaction vessel, to form a resin that is further polymerizable by solid-state polymerization and moldable to form a PET article that is transparent and substantially free of yellow or blue coloration.
In another aspect, the present invention relates to a method of making a PET resin, comprising the steps of:
reacting dimethyl terephthalate (DMT) or terephthalic acid (TA) with ethylene glycol (EG), in the presence of a catalytically effective amount (if needed) of an esterification catalyst, such as manganese acetate, zinc acetate, or cobalt acetate, to produce monomers;
mixing the monomer with a preblended additive solution comprising cobalt acetate and polyphosphoric acid (PPA) to form a preblended monomer composition; and
subjecting the preblended monomer composition to a polycondensation reaction in the presence of a polycondensation catalyst such as antimony trioxide, antimony acetate, titanium butylate, titanium glycolate or titanium oxalate.
Another aspect of the invention relates to a method for mitigating color and haze development in the manufacture of bottles molded from a PET resin, said method comprising the steps of:
reacting a dicarboxylic acid or its ester equivalent with a diol at a temperature from about 150xc2x0 C. to about 270xc2x0 C. to start the reaction and form monomer;
preparing an additive complex or composition comprising a cobalt compound and a phosphorus compound;
adding the additive complex or composition before the polycondensation reaction;
subjecting the monomer and additive composition to a polycondensation reaction in the presence of a polycondensation catalyst at a temperature in the range of from about 250xc2x0 C. to about 300xc2x0 C., to form the PET resin; and
molding the PET resin to form a bottle of mitigated color and haze development.
The invention relates in a further aspect to a cobalt-phosphorus complex or compound for mitigating the development of color and haze in an article molded from a polyester resin, such cobalt-phosphorus complex or compound comprising a cobalt compound and a phosphorus compound.
Another aspect of the invention relates to a preblended additive comprising cobalt acetate and polyphosphoric acid (PPA), useful in the above-described PET process.
Yet another aspect of the invention relates to a molded PET article formed of a PET resin produced by a DMT or TA process, wherein the PET resin has been treated by a cobalt-phosphorus complex or composition comprising the reaction product of a cobalt compound and a phosphorus compound.
Other aspects, features and embodiments of the invention will be more fully apparent from the following disclosure and appended claims.
The present invention contemplates an improved additive and process for the production of polyester resin and particularly for PET resin for the manufacture of plastic articles, e.g., such as bottles for containing beverages.
All percentages used herein are mole % values, and all part-per-million (ppm) concentrations used herein are weight-based, unless-otherwise expressly specified.
Polyesters are produced from the reaction of one or more dicarboxylic acids (or their ester equivalents) with one or more diols. Suitable dicarboxylic acids are terephthalic acid, isophthalic acid, hemimellitic acid, trimellitic acid, trimesic acid, fumaric acid, maleic acid, sebacic acid, succinic acid, adipic acid, naphthalenedicarboxylic acid, cyclohexanedicarboxylic acid, bibenzoic acid, or a mixture of any of these. The ester equivalents may be, for example, a dimethyl, diethyl, or dipropyl ester of the dicarboxylic acid. Suitable diols are ethylene glycol, diethylene glycol, triethylene glycol, dipropylene glycol, propane diol, butane diol, cyclopentane diol, cyclohexane diol such as cyclohexane dimethanol, and neopentyl diol, or a mixture of these. The term xe2x80x9cPETxe2x80x9d as used herein is intended to be broadly construed to include xe2x80x9cpolyethylene terephthalate,xe2x80x9d and any similar polyesters derived from the reaction of terephthalic acid with one or more diols, such as di-glycol or di-glycol ether, or from the reaction of terephthalic acid diester, for example, dimethyl terephthalate (DMT) and similar diesters, with a di-glycol or di-glycol ether.
The term PET as used herein is also intended to be broadly construed to further include a combination of PET as hereinabove defined, and up to about 20% of non-polyester polymers, such as for example polyolefins, polyamides, and similar compounds known to those skilled in the art as capable of being combined with PET.
The term PET as used herein is intended to be broadly construed to include polyethylene terephthalate per se, and copolymers thereof, such as for example polyethylene-terephthalate-isophthalate, or polyethylene-terephthalate-naphthalate. Also it is sometimes desirable to produce colored bottles or bottles with other desired characteristics, and thus using other conventionally known additives such as pigments, dyes, fillers, branching agents, reheating agents, etc. and these additives, when employed in the resin, are also broadly construed to be within the meaning of PET.
In the present invention, if the DMT process is used, then an ester interchange catalyst must be used. Potentially useful ester interchange catalysts include compounds containing one or more of cobalt, zinc, or manganese. In one embodiment of the present invention, DMT is reacted with EG in a mole ratio of from about 1.9 to about 2.4 in the ester interchange reaction, in the presence of an appropriate catalyst at a level of from about 45 to about 90 ppm.
It is known by those skilled in the art that in the DMT process the ester interchange catalyst must be sequestered or made catalytically inactive by the addition of phosphorous at the termination of the esterification reaction. The phosphorus compound forms a complex with the active catalysts (sequestering the ester interchange catalyst does not affect any polycondensation catalyst that may be present). If the catalyst employed in the ester interchange reaction is not sequestered, then the polymer can readily degrade and exhibit an undesired yellow color. If a phosphorus compound is added to sequester the ester interchange catalyst, it is added separately, after the addition of the ester interchange catalyst, and near the end of the esterification reaction or just before the polycondensation reaction. A typical temperature range for the ester interchange reaction is from about 150xc2x0 C. to about 270xc2x0 C. In a preferred embodiment of the invention, the ester interchange temperature does not exceed about 230xc2x0 C.
In the TA process, no sequestering is necessary because no catalyst is used in the transesterification reaction. For the TA process, the TA is reacted with EG in a mole ratio of from about 1.0 to about 1.4.
The monomer, formed by either process, then undergoes a polycondensation (PC) reaction to yield the desired polyester resin. The polycondensation step (PC) is carried out under high temperature (about 250xc2x0 C. to about 300xc2x0 C.) and under vacuum (0.1 to 3 mm torr). Typical catalysts for the polycondensation reaction are antimony acetate, antimony trioxide, titanium butinate, titanium oxylate, or mixtures of these.
In a preferred embodiment of this invention, a phosphorus compound such as polyphosphoric acid (PPA) is rapidly reacted with a cobalt compound (by quickly dumping the phosphorus compound into the cobalt compound) such as cobalt acetate at room temperature with stirring or agitation. After the quick addition of the phosphorus to the cobalt, the preblend is mixed for about 2 hours. This is considered xe2x80x9cfastxe2x80x9d preblending. In another embodiment, the phosphorus compound is added to the cobalt compound more slowly over a period of 5 to 90 minutes (the result seems not to differ) with stirring or agitation for a total of 2 hours. This is called xe2x80x9cslowxe2x80x9d preblending. If the preblended complex or compound is to be stored, then it is preferred to have nitrogen in the headspace of the storage vessel to minimize water absorption. High water absorption causes precipitates to form.
The cobalt compounds suitable for the present invention are cobalt acetate, cobalt benzoate, cobalt arsenate, cobalt formate, or mixtures of these. Preferred is cobalt acetate because of its easy in mixing with glycol and because its readily available. The phosphorus compounds suitable with the present invention are phosphoric acid, polyphosphoric acid, and phosphorous acid, or a mixture of these. Although other phosphorus compounds may be suitable, these listed are the most preferred because they easily mix with glycol compounds and thus are easily blended with the cobalt acetate. Of these preferred compounds, the most preferred is PPA.
More specifically, a solution of cobalt acetate in virgin ethylene glycol is prepared by adding cobalt acetate to ethylene glycol. Confirmation that the cobalt acetate is totally dissolved in the glycol is noted by the presence of any particles in the glycol. This typically takes one hour at a minimum. Preferably the phosphorus compound may also be mixed with ethylene glycol, for example. The preferred phosphorus compound is PPA, which readily blends with cobalt acetate. However, it has been observed that using neat PPA results in some precipitates, which must be filtered out. On the other hand, when PPA is blended with glycol, there appears to be little or no visible precipitates. So it is still advantageous to mix the phosphorus compound with glycol. This also permits easy blending of the two-glycol mixtures of cobalt and phosphorus to form the complex or compound. Typically the cobalt-phosphorus complex or compound is prepared with a 30 mole % excess of phosphorus. The preblended cobalt-phosphorus complex or compound is added at the end of the ester interchange (DMT process) or direct esterification (TA process), or at the beginning of polycondensation.
The preblended cobalt-phosphorus complex or compound may suitably be added, in a batch process, over the course of 5 to 10 minutes at the end of the ester interchange/direct esterification, or at the beginning of the polycondensation reaction. Of course, in a continuous process the preblended solution is continuously added. At the end of polycondensation, the vacuum is let down and the resulting amorphous resin is less hazy and more neutral in color.
The resulting amorphous or semicrystaline pellets may then be subjected to solid-state polymerization (SSP). This solid-state polymerization reaction, which further polymerizes the resin, involves heating the pellets below their melting point to a temperature in the range of from about 200xc2x0 C. to about 240xc2x0 C. under partial vacuum or in an inert gas flow. The solid-state polymerization reaction can be carried out using fluidized bed reactors, in tumbling chip reactors, or in fixed bed reactors. The typical intrinsic viscosity, IV, of the polymer may be in the range of from about 0.7 to about 0.9 dl/g. The SSP chip can then be melt extruded and molded into containers.